1. Field of the Invention
The present invention is directed to extracts from the leaves and flowers of the creosote bush (Larrea tridentata, Zygophyllaceae) having anti-HIV activity. The extracts of at least L. tridentata contain a variety of tricyclic compounds, including cyclolignan compounds, which are also the subject matter of the present invention. The invention also embraces derivatives and isomers of the tricyclic compounds found in the extracts. Finally, the present invention also encompasses methods for treating retroviral infections, especially HIV infection, by administering the extract and/or purified tricyclics or cyclolignans to an HIV-infected subject.
2. Description of the Related Art
Articles of the scientific literature cited throughout this application are hereby incorporated by reference in their entirety by such citation.
It is becoming clear that control or cure of HIV infection in a subject will require a multifacetted approach, which partly relies upon inhibition of the HIV replication cycle. Furthermore, it is clear that a several aspects of viral replication will have to be attacked. For instance, it has recently been shown that a regimen of multiple drugs, such as inhibitors of reverse transcriptase (RT), AZT, ddI, and the protease inhibitor, 3TC, has proved more effective in controlling the virus than administration of any one of these drugs alone.
It has been shown that the creosote bush and related species contain active principles with antimicrobial and antiviral activity (Ayres et al., Ch. 3, p. 85 in Lignans: Chemical, Biological and Clinical Properties--(Chemistry and Pharmacology of Natural Products), Ayres and Loike, Eds., c. 1990 by Cambridge University Press, New York,).
Tat is a transactivator of human immunodeficiency virus (HIV) gene expression. Tat and Rev are indispensable viral regulatory factors for HIV gene expression. Tat acts by binding to the TAR RNA element and activating transcription from the long terminal repeat (LTR) promoter of the HIV-1 proviral DNA.
It has been shown that upon infection with AIDS virus, basal viral gene transcription is initiated. Activation of viral genes occurs upon antigenic stimulation and results in a burst of viral replication. Expression of the tat gene has been shown to result in several thousand-fold increased levels of HIV LTR-driven gene expression at both the transcriptional and postranscriptional levels.
The Tat protein stabilizes elongation of transcription complexes and has also been shown to be involved in transcription initiation. Previous studies have shown that Tat mediates reduction of antibody-dependent T cell proliferation, contributing substantially to the failure of the immune response. Tat also directly stimulates the growth of Kaposi's sarcoma cells. Tat is indirectly involved in HIV-related dementia in patients with advanced AIDS.
Since Tat has no apparent cellular homologs, this strong positive regulator has become an attractive target for the development of anti-AIDS drugs (see FIG. 1). Currently available AIDS drug such as HIV reverse transcriptase inhibitors (AZT, DDI) or the protease inhibitor (3TC) target viral gene products (enzymes and specific viral proteins). These drugs are capable of suppressing the replication of a wild-type AIDS virus. However, they are ineffective in destroying a small pool of coexisting mutant viruses which continue to replicate at a high rate in the presence of these drugs. An inhibitor which targets Tat transactivation and viral gene transcription at the level of integrated proviral DNA will arrest the virus at an early stage (Hsu et al., Science, 254:1799-1802, 1991). Because replication of the viral genome is suppressed, virtually no viral resistance to the drug should develop.
Efforts aimed at the elucidation of factors which control gene expression at the transcriptional and post-transcriptional levels in host eukaryotes have recently made possible quantitative assessment of Tat function (Sim, Ann. N.Y. Acad. Sci., 616:64-70, 1990).
The HIV-1 integrase (HIV-1 IN) mediates the integration of the transcribed viral DNA into the host chromosome, a crucial step in the HIV replication cycle. This enzyme has no known cellular homolog, and relatively simple in vitro assay systems have enabled functional characterization and the screening of specific inhibitors of the HIV-1 IN. Therefore HIV-1 IN has also become a major target for the development of specific anti-AIDS drugs.